In This Issue
The Bridge: 50th Anniversary Issue
January 7, 2021 Volume 50 Issue S
This special issue celebrates the 50th year of publication of the NAE’s flagship quarterly with 50 essays looking forward to the next 50 years of innovation in engineering. How will engineering contribute in areas as diverse as space travel, fashion, lasers, solar energy, peace, vaccine development, and equity? The diverse authors and topics give readers much to think about! We are posting selected articles each week to give readers time to savor the array of thoughtful and thought-provoking essays in this very special issue. Check the website every Monday!

Accelerating Growth of Solar Energy

Thursday, December 24, 2020

Author: Sarah R. Kurtz

In 1970 silicon solar cells were used for powering satellites but were too expensive for terrestrial applications. Now they are the fastest-growing source of bulk electricity in many locations, accounting for an impressive 43 percent of worldwide net electricity-generating capacity expansion in 2019 (figure 1, pie chart; REN21 2020).

In 2008 the NAE named as one of its 14 Grand Challenges for Engineering[1] “Make Solar Energy Economical.” At the time, solar panels cost about $4/W (Feldman et al. 2012), far too expensive to be a practical power source for the world. In 2020 the cost can be less than $0.20/W and solar electricity power purchase agreements have been reported for prices as low as 1.35 cents/kWh (Bellini 2020)—less than half the average 2019 US wholesale price of electricity (3.8 cents/kWh).[2]

Growth in Solar Capacity, Day and Night

Solar deployment has grown consistently faster than predicted (Haegel et al. 2017), apparently accelerated by a positive feedback loop in which public enthusiasm drove policy incentives, which drove deployment and lower costs, which further increased enthusiasm, in a repeating cycle (lower circle in figure 1). Growth between 2005 and 2015 was so rapid[3] (a factor of 63) that if the same relative rate continued solar electricity generation would approach the entire world electricity demand by about 2025 (dotted green line in figure 1).

Kurtz figure 1.gif

Having delivered 2.8 percent of the world’s electricity in 2019 (REN21 2020), solar energy has now entered a new era in which the primary challenge is to identify ways to use variable solar electricity to provide power at every moment of the day and night. Fortunately, impressive advances in lithium ion batteries in recent years are enabling use of solar electricity after the sun sets. The Colorado Public Utility Commission recently accepted an Xcel Energy proposal to reduce costs by prematurely retiring a coal-fired plant and replacing it with a combination of solar plus batteries (Jackson 2020). This is a ground-breaking instance of solar and storage unseating fossil fuels, competing directly on cost and at the request of a utility.

Wind electricity has also decreased in cost and wind as a resource is often strongest at times when solar resource falls short (at night, in winter, and during storms). Large deployment of wind and solar, coupled in a smart way with expanded deployment of technologies available today, can create a cost-effective and highly reliable electrical grid with 90 percent of electricity generated by carbon-free sources (e.g., Phadke et al. 2020). Strong policy action could enable this 90 percent milestone by 2035 (Phadke et al. 2020).

The Role of Positive Feedback

Just as the solar industry grew much faster than expected in recent decades, it may be poised for further impressive growth driven by an additional positive-feedback mechanism involving renewable electricity, electrification, and tools that improve grid flexibility (upper right circle of figure 1). Electrification of the transportation and heating sectors using electric vehicles and heat pumps not only reduces reliance on fossil fuels but also increases efficiency, reducing the total energy needed (Kurtz et al. 2020).

Key elements of the flexible grid will include large storage capabilities, transmission, and demand management to maintain tight balance between electricity supply and demand at every moment of the day. The feedback makes the job easier; for example, electrification of transportation will introduce flexible loads (vehicle charging) that may be shifted to times of electricity abundance.

New and updated technologies such as
liquid air, gravity, thermal, and geomechanical storage may provide scalable
energy storage options.

Continuing the enthusiasm-driven positive feedback loop and adding coordinated implementation of electrification and flexible grid technologies will accelerate the transition while reducing cost.

New Technology Needs and Research Opportunities

Reaching the goal of 100 percent of electricity generation without using fossil fuels will require new technology. For example, the seasonal fluctuations of energy demand are problematic without some form of seasonal storage or long-distance transmission.

It may be that hydrogen coupled with fuel cells could solve the problem by using surplus electricity to split water into hydrogen and oxygen, then using the hydrogen later to regenerate the electricity. In addition to chemical storage like hydrogen, new and updated technologies such as liquid air, gravity, thermal, and geomechanical storage may provide scalable storage options.

The best pathway to a 100 percent zero-carbon grid is under debate, but many studies have identified possible pathways to achieve this by 2050. Coupling this energy transition with electrification will reduce carbon emissions associated with transportation and heating and will accelerate the conversion of the grid by increasing both the demand for renewable electricity and the flexibility of the grid (e.g., assuming that electric vehicles can charge during times of abundant electricity).

While today’s photovoltaic technology has advanced enough to drive the grid, it is still in its infancy. Just in the last 5 years, the silicon-based solar industry has changed product lines, shifting to advanced mono-crystalline silicon solar cells and to bifacial cells and modules. Companies are developing modules that deliver more than 500 W. In parallel, cadmium telluride modules are now made larger and more efficient.

Debate persists about the technical trajectory of photo-voltaic technology. The power generated by a single solar panel continues to increase and there is a vision of practical tandem solar cells with substantially higher efficiencies. Tandem cells capture high-energy photons (mostly visible light) with a high-band-gap solar cell and lower-energy photons (mostly near-infrared light) with a lower-band-gap solar cell. Some companies are starting to commercialize tandems using higher--band-gap perovskite cells coupled with the low-band-gap silicon tandems. In addition, solar panels are being engineered to replace building materials, and they can be integrated into cars and be made with any color to better blend into the environment.

Solar energy has made amazing progress over the last 50 years, enabling visions of a new era of growth accelerated by coordinating solar’s growth with electrification and flexible grid advancements. When The Bridge celebrates its 100-year anniversary, solar may be the world’s largest source of energy.


Thanks to James Bernard and Rhonda Bailey for useful comments.


Bellini E. 2020. Abu Dhabi’s 1.5 GW tender draws world record low solar bid of $0.0135/kWh. PV Magazine, Apr 28.

Feldman D, Barbos G, Margolis R, Wiser R, Darghouth N, Goodrich A. 2012. Photovoltaic (PV) Pricing Trends: Historical, Recent, and Near-Term Projections (Technical Report DOE/GO-102012-3839). Golden CO: National Renewable Energy Laboratory and Berkeley CA: Lawrence Berkeley National Laboratory.

Haegel NM, Margolis R, Buonassisi T, Feldman D, Froitzheim A, Garabedian R, Green M, Glunz S, Henning H-M, -Holder B, and 11 others. 2017. Terawatt-scale photo-voltaics: Trajectories and challenges. Science 356(6334):141–43.

Jackson AK. 2020. Xcel’s Colorado energy plan. Presentation, Colorado Renewable Energy Society, Feb 17. Online at

Kurtz S, Leilaeioun A, King RR, Peters IM, Heben MJ, Metzger WK, Haegel NM. 2020. Revisiting the terawatt challenge. MRS Bulletin 45(3):159–64.

Phadke A, Paliwal U, Abhyankar N, McNair T, Paulos B, Wooley D, O’Connell R. 2020. 2035: The Report. -Goldman School of Public Policy, University of -California, Berkeley.

REN21. 2020. Renewables 2020 Global Status Report. Paris: REN21 Secretariat.



FIGURE 1 Graph of historical world electricity generation by technology and pie chart of net electricity-generating capacity expansions in 2018–19. The lower positive feedback loop fueled surprising growth of solar (green line) in 2005–15. The dotted green line shows how continuation of that growth would enable solar electricity to meet most of the world’s electricity needs by about 2025, a level that would be practical only if world electricity demand increased by massive electrification as shown schematically by the dotted black line and as reflected by the positive feedback mechanisms shown in the top right loop. Based on data from REN21 (2020) and US Energy Information Administration, International data: Electricity (, accessed Jul 4).



[2]  Derived by averaging the data for 2019 at (Jul 4).

[3]  US Energy Information Administration, International data: Electricity (, accessed Jul 4).

About the Author:Sarah Kurtz (NAE) is a professor in the Materials Science and Engineering Department at the University of California, Merced.